DC to DC converters are frequently employed to convert relatively low voltage DC sources into high voltage DC, suitable for applying voltages to DC loads, such as electron tube electrodes. For example, such converters are employed for supplying up to 4,500 volts between collector and cathode electrodes of high voltage traveling wave tubes. Such power supplies also desirably are capable of deriving DC voltages at several different levels and are responsive to a wide range of voltages from a DC source, and to supply medium power levels, e.g., 50-200 watts to the load. Because of the possible wide input voltage variation, it is necessary for such converters to be regulated. It is also important for the DC source, which is frequently a battery, to be isolated from the load and circuitry driving the load.
It is very important for high voltage output circuitry of such converters to have no high voltage inductor. A high voltage inductor frequently precludes the capability for multiple outputs, as required for high voltage electron tube energization, and adversely affects converter cost and size. It is also desirable for the converters to have relatively high efficiency, i.e., in excess of 85%, and to minimize ripple of input current derived from the DC source, particularly when the source is a battery. It is also desirable, but not necessary, for the load voltage ripple to be small, to minimize output filtering.
One class of converter that has been developed employs a transformer having primary winding means connected to the DC source and a secondary winding connected to rectifier circuitry for driving the high voltage load. Switching circuitry is connected to the primary winding means to chop the DC voltage from the input source into AC which is coupled by the transformer and secondary winding to the load via the rectifier, which converts the secondary winding AC into DC. Frequently, the switching circuitry is activated at a fixed frequency, with variable duty cycle. The output voltage is related to the input voltage, the duty cycle, and turns ratio between the primary winding means and the secondary winding. By varying the duty cycle, the DC voltage is regulated to compensate for variations of input voltage. Exemplary of circuits having this configuration are the forward converter, buck regulator with push-pull inverter, boost regulator with push-pull inverter, Venable converter, flyback or buck/boost converter, Sepic converter, and C'uk converter.
In these types of converters, it is desirable to provide flux cancellation in the transformer, as well as to minimize the number of switches and the number of elements having magnetic cores, which are heavy, require substantial space, and expensive. In addition, it is desirable for the current from the DC source to be automatically limited in response to the load being inadvertently substantially increased, e.g., by being short circuited. If such automatic current limiting is not provided the source may be effectively discharged or the converter damaged.
Of the previously mentioned prior art converters, the forward and Cu'k converters require high voltage inductors, and thereby are inappropriate for deriving high output voltages. The requirement for no high voltage inductor is so great that the Cu'k converter is inappropriate even though it otherwise has a higher figure of merit than any of the other named converters. While the flyback and Sepic converters do not require a high voltage inductor and are capable of handling wide ranges of input voltage and are current limited, these circuits do not provide transformer flux cancellation and have efficiencies less than 85%. In addition, the flyback converter has relatively large input current ripple. The flyback and Sepic circuits only use a single switch; the flyback circuit employs one magnetic core element, while the Sepic circuit employs two such cores. Both of these circuits also suffer from relatively high output voltage ripple and thereby have a figure of merit which is slightly lower than that of the Cu'k converter. The other named converters have figures of merit which are still lower than those of the flyback and Sepic converters.
It is, accordingly, an object of the present invention to provide a new and improved DC to DC converter particularly adapted for high voltage, multiple outputs.
Another object of the invention is to provide a new and improved DC to DC converter having a high figure of merit without employing a high voltage inductor.
An additional object of the invention is to provide a new and improved DC to DC regulated high voltage converter.
Another object of the invention is to provide a new and improved DC to DC regulated high voltage converter, particularly suitable for deriving multiple outputs from a DC source having a relatively low to medium DC voltage.
A further object of the invention is to provide a DC to DC high voltage converter, particularly suited for multiple outputs, wherein the converter does not include a high voltage inductor, is relatively efficient, is capable of handling a wide range of input voltages, has low input current ripple, provides voltage gain by controlling duty cycle, provides flux cancellation in a transformer, is automatically current limited, and employs a small number of switches as well as a small number of magnetic core elements.